Effect of Osmotic Pressure in the Solvent Extraction Phase on BSA Release Profile from PLGA Microspheres

This study investigated the influence of osmotic pressure in the organic solvent extraction phase on release profile of bovine serum albumin (BSA) from poly(lactide-co-glycolide) (PLGA) microspheres. BSA-loaded PLGA microspheres with a target load of 10% were prepared by a double emulsion phase separation method. All the microsphere batches were fabricated in the same conditions except that in the organic solvent (CH2Cl2) evaporation step. Different concentrations of NaCl (0, 1.8, and 3.6%) or sucrose (20%) were used to generate a range of osmotic pressures in the extraction aqueous phase. These microspheres were characterized for incorporation efficiency, surface and internal morphology, particle size, protein stability, and in vitro release. The microspheres were spherical with particle size ranging from 16.8 to 27.8 microns. Higher osmotic pressure resulted in a denser internal structure although similar nonporous surface morphology was observed with all batches. No significant difference in encapsulation efficiency existed from batch to batch (87-94%). Sodium dodecyl sulfate-polyamide gel electrophoresis showed that BSA integrity was well retained. The release profile of the batch prepared with only water as the continuous (solvent extraction) phase exhibited a 79% burst release in the first 24 hr followed by a plateau and then a little release after 21 days. In the presence of NaCl or sucrose, the burst effect significantly decreased with increase in osmotic pressure in the extraction aqueous phase, which was then followed by sustained release for 35 days. A mass balance was made when the release terminated. Therefore, in the organic solvent extraction and evaporation step, increasing the osmotic pressure in the aqueous phase both reduced the burst release from the microspheres and improved the subsequent sustained release profile.